This invention relates to a system for providing an electron beam on a fixed axis from a selected one of two cathodes equidistant from the axis and in the same plane passing through the axis. Upon selection of one cathode, it is turned on while the other cathode is off, and vice versa, to maintain an electron beam from either cathode.
For long duration aerospace missions, and for other applications, there is a need to maintain the operation of electron beam instruments, such as a scanning electron microscope (SEM), despite the failure of the electron beam cathode. A miniaturized, light weight, low powered SEM and x-ray analysis system designated as the Scanning Electron Microscope and Particle Analyzer (SEMPA) instrument for space-flight applications has been under development for the last eight years. One proposed application of the SEMPA instrument is for the NASA Marner Mark II Comet Rendezvous/Asteroid Flyby (MMII CRAF) mission for launch in the 1990's.
During the CRAF mission, the spacecraft will be in the vicinity of the comet for about three years. In that interval the SEMPA is expected to sample the dust leaving the comet surface at least 50 times, and spend a total of about 1000 hours doing imaging and x-ray analysis of the collected dust.
Such a long duration mission places great demands on the reliability and durability of its scentific instrument packages. In particular, for SEMPA, the x-ray detector and cathode systems have been the subject of special concern. Since the probability of failure of such parts is significant, it is imperative that redundancy be provided for those parts. The cathode of an electron beam source is of particular concern for it requires careful examination of possible techniques for achieving cathode redundancy. It would be desirable to have two cathodes for alternate use in the SEMPA instrument, each designed to provide an electron beam on a fixed electron optical axis for about 1000 hours of operation, or one of lanthanum hexaboride (LaB.sub.6) designed for a long life, but unstable source of electrons, and the other of tungsten designed for a stable, but short-life source of electrons.
There are two practical techniques for providing redundant cathodes. One is to provide a mechanical system, such as a shuttle or carousel, that could place either of two cathodes on the electron optical axis of the instrument. Such a system is certainly feasible, but would probably be heavy, expensive, and have its own reliability concerns. The alternative approach is to have two fixed cathodes, and use beam deflection fields to steer the electron beam from each onto a fixed electron optical axis. When one cathode fails, the alternate is turned on, and the necessary beam deflection fields are activated to steer the emitted electrons onto the fixed electron optical axis. The present invention pursues this alternate approach.